>How I can know the number of Items selected in a multiple selection ListBox, I know how to count the entiere listbox using .ListCount but I need to know how many file I selected.
You can loop throu the listbox otems and check Selected property.

   lnSelCount = 0
   FOR nCnt = 1 TO ThisForm.lstListBox1.ListCount
      IF ThisForm.lstListBox1.Selected(nCnt)  && Is item selected?
         lnSelCount = lnSelCount + 1
      ENDIF
   ENDFOR